The present invention relates generally to the field of solid-state electrochemical devices, and more particularly to a novel structure for an electrode/electrolyte/electrode with an unusual an unexpected electrochemical performance when used as solid oxide ful cells.
Solid-state electrochemical devices are often implemented as cells including two porous electrodes, the anode and the cathode, and a dense solid electrolyte and/or membrane, which separate the electrodes. For the purposes of this application, unless otherwise explicit or clear from the context in which it is used, the term “electrolyte” should be understood to include solid oxide membranes used in electrochemical devices, whether or not potential is applied or developed across them during operation of the device. In many implementations, such as in fuel cells and oxygen and syn-gas (H2+CO) generators, the solid membrane is an electrolyte composed of a material capable of conducting ionic species, such as oxygen ions, or hydrogen ions, yet has a low electronic conductivity. In other implementations, such as gas separation devices, the solid membrane is composed of a mixed ionic electronic conducting material (“MIEC”). In each case, the electrolyte/membrane must be dense and pinhole free (“gas-tight”) to prevent mixing of the electrochemical reactants. In all of these devices a lower total internal resistance of the cell improves performance.
The ceramic materials used in conventional solid-state electrochemical device implementations can be expensive to manufacture, difficult to maintain (due to their brittleness) and have inherently high electrical resistance. The ionic resistance may be reduced by operating the devices at high temperatures, typically in excess of 900° C. However, such high temperature operation has significant drawbacks with regard to the device maintenance and the materials available for incorporation into a device, particularly in the oxidizing environment of an oxygen electrode, for example.
The preparation of solid-state electrochemical cells is well known. For example, a typical solid oxide fuel cell (SOFC) is composed of a dense electrolyte membrane of a ceramic oxygen ion conductor, a porous anode layer of a ceramic, a metal or, most commonly, a ceramic-metal composite (“cermet”), in contact with the electrolyte membrane on the fuel side of the cell, and a porous cathode layer of a mixed ionically/electronically-conductive (MIEC) metal oxide on the oxidant side of the cell. Electricity is generated through the electrochemical reaction between a fuel (typically hydrogen produced from reformed methane) and an oxidant (typically air). This net electrochemical reaction involves charge transfer steps that occur at the interface between the ionically-conductive electrolyte membrane, the electronically-conductive electrode and the vapor phase (fuel or oxygen). The contributions of charge transfer, mass transfer (gas diffusion in porous electrode), and ohmic resistance to the total internal resistance of a solid oxide fuel cell device during electronic and ionic current flow can be significant. Moreover, in typical device designs, a plurality of cells are stacked together and connected by one or more interconnects. Resistive loss attributable to these interconnects can also be significant.
In work reported by de Souza et al. (de Souza, S.; Visco, S. J.; De Jonghe, L. C. Reduced-temperature solid oxide fuel cell based on YSZ thin-film electrolyte. Journal of the Electrochemical Society, vol. 144, (no. 3), Electrochem. Soc, March 1997. p.L35-7.7), a thin film of yttria stabilized zirconia (YSZ) is deposited onto a porous cermet electrode substrate and the green assembly is co-fired to yield a dense YSZ film on a porous cermet electrode. A thin cathode is then deposited onto the bilayer, fired, and the assembly is tested as an SOFC with good results. In work reported by Minh (Minh, N. Q. (Edited by: Dokiya, M.; Yamamoto, O.; Tagawa, H.; Singhal, S. C.) Development of thin-film solid oxide fuel cells for power generation applications. Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells (SOFC-IV), (Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells (SOFC-IV), Proceedings of Fourth International Symposium Solid Oxide Fuel Cells, Yokohama, Japan, 18–23, Jun. 1995.) Pennington, N.J., USA: Electrochem. Soc, 1995. p. 138–45), a similar thin-film SOFC is fabricated by tape calendaring techniques to yield a good performing device. However, these Ni-YSZ supported thin-film structures are mechanically weak, and will deteriorate if exposed to air on SOFC cool-down due to the oxidation of Ni to NiO in oxidizing environments. Also, nickel is a relatively expensive material, and to use a thick Ni-YSZ substrate as a mechanical support in a solid-state electrochemical device will impose large cost penalties.
Solid-state electrochemical devices are becoming increasingly important for a variety of applications including energy generation, oxygen separation, hydrogen separation, coal gasification, and selective oxidation of hydrocarbons. These devices are typically based on electrochemical cells with ceramic electrodes and electrolytes and have two basic designs: tubular and planar. Tubular designs have traditionally been more easily implemented than planar designs, and thus have been proposed for commercial applications. However, tubular designs provide less power density than planar designs due to their inherently relatively long current path that results in substantial resistive power loss. Planar designs are theoretically more efficient than tubular designs, but are generally recognized as having significant safety and reliability issues due to the complexity of sealing and manifolding a planar stack.
Thus, solid state electrochemical devices incorporating current implementations of these cell designs are expensive to manufacture and may suffer from safety, reliability, and/or efficiency drawbacks. Some recent attempts have been made to develop SOFCs capable of operating efficiently at lower temperatures and using less expensive materials and production techniques. Plasma spray deposition of molten electrolyte material on porous device substrates has been proposed, however these plasma sprayed layers are still sufficiently thick (reportedly 30–50 microns) to substantially impact electrolyte conductance and therefore device operating temperature.
Accordingly, a way of reducing the materials and manufacturing costs and increasing the reliability of solid state electrochemical devices would be of great benefit and, for example, might allow for the commercialization of such devices previously too expensive, inefficient or unreliable.
Thin films as disclosed here in find use in electrochemical devices and as barrier coatings when deposited on dense substrates. Applications include but are not limited to: fuel cells, oxygen separation, hydrogen separation, mixed ionic electronic thin film devices, sensors, magnetic films (perovskites), wear resistant applications, barrier coatings, oxidation resistant coating and thermal barrier coatings.
Prior art structures are constructed by a co-firing method whereby an electrolyte was cofired onto a substrate, typically an electrode. A counter electrode was then bonded onto the resultant structure. Co-firing of a bi-layer rather than a tri-layer was done for several reasons. First, the temperature required for sintering of the thin-film electrolyte was high enough that reaction between the thin film electrolyte and counter electrode would lead to a resistive interface. Second, it was believed that in order to achieve good electrode performance the surface area of the counter electrode had to be maximized. High surface area counter electrodes were fabricated by applying a high surface area powder to a fired bilayer and firing the counter electrode at a temperature sufficient for bonding, but not high enough for significant densification of the electrode. The co-firing method suffers from other disadvantages. By firing each electrode separately, uniform neck size could be difficult to achieve in each electrode/electrolyte interface, and subsequently device performance could suffer. Also, dual firing exposes one electrode to two sintering processes, and the attendant disadvantages therein.
Porous composite electrodes in contact with a dense electrolyte membrane and methods of making them are known in the art and disclosed in U.S. Pat. No. 5,670,270, the contents of which are hereby incorporated by reference in its entirety. U.S. Pat. No. 3,377,203 discloses a method for producing fuel cells of solid electrolyte and ceramic oxide electrode layers by sintering the electrode layers to the electrolyte. U.S. Pat. No. 4,767,518 discloses a solid oxide electrode (anode) made of metal particles that are immobilized by stabilized zirconia which may also contain praseodymium (Pr). The Pr may be added in the form of a solution. U.S. Pat. No. 4,885,078 discloses an electrochemical device which may be a solid oxide cell which comprises a porous electrode containing a deposit of metal oxide or metal salt capable of forming metal oxide upon heating, where the metal may be Pr. U.S. Pat. No. 5,021,304 discloses a method of coating a separate electronically conducted layer on a porous electrode having the steps of applying a mixture of metal salts including nitrates to the electrodes with a surfactant, and heating to form the oxides. Pr oxide is included in a list of dopant oxides which may be used.
Achieving highly dense, crack-free thin films on rigid substrates is of considerable technological importance to a wide variety of fields from the packaging of microelectronic circuits-to high temperature solid oxide fuel cells. Thin films are used for a variety of optical and protective uses.
Methods involving sol-gel technology can produce dense, uniform films, but there is a thickness limitation and these are agenerally unsuitable for porous substrates. Brinker and Scherer (Sol-Gel Science, Academic Press, Inc. 1990) state that there is an observed thickness limit of approximately 1 μm. These thin films suffer from cracking problems during drying or sintering when thickneses beyond 1 μm are attempted. Films thicker than about 1 μm have only been dried succesfuly by incorporating organics into the network to rovide extra compliance (rubberiness) to prevent cracking. This problem is exacerbated when the films are deposited on porous substrates, see Kueper et al. Solid State Ionics 52 (1992) 251–259, the contents of which are incorporated by reference. Other methods involving vacuum technology such as CVD, EVD etc can produce dense, uniform layers on dense or porous substrates. However, the drawback to this technology is the cost of the process.
In the prior art techniques, the importance of the starting green density was not realized, and the effect that the initial compaction would have on the final resultant density.
Porous coatings deposited onto a substrate and sintered are known in the art. U.S. Pat. No. 6,358,567 discloses a method for producing coatings by colloidal spray. U.S. Pat. No. 6,270,642 B1 discloses a method for depositing an electrolyte material on a porous substrate by electrophoretic deposition. Microlaminated composite articles are disclosed in U.S. Pat. No. 5,350,637. A method for reducing the shrinkage during firing of ceramic bodies is disclosed in U.S. Pat. Nos. 5,474,741 and 5,102,720. Co-sintered multilayered fuel cells are disclosed in U.S. Pat. No. 5,057,362. However, an electrochemical device wherein a porous ceramic layer is deposited on a substrate, compacted prior to firing to high density, and wherein the substrate does not shrink during firing of the ceramic thin film is not known in the art. The contents of the above referenced patents are hereby incorporated by reference in their entirety.
The prior art has attempted to solve the aforemetioned problems, but with limited success. Yao et al., Sensors and Actuators A 71 (1998) 139–143, the contents of which are incorporated by reference in their entirety, have applied isostatic compresion to PZT thick-film actuators in situ on an Al2O3 substrate.